the nacl in the solution of the visine will invalidate the test and therefore cause the test to read negative for thc. those tests have such a huge margin of error and they cost whoever is footing the bill so much that they will not suspect nor retest you. and if they do..... do the visine thing again. but it is illegal so be good or be good at it.

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the efficient man does the job right
the effective man does the right job